A former United Nations police officer is suing a British security firm over claims that it covered up the involvement of her fellow officers in sex crimes and prostitution rackets in the Balkans.

Kathryn Bolkovac, an American policewoman, was hired by DynCorp Aerospace in Aldershot for a UN post aimed at cracking down on sexual abuse and forced prostitution in Bosnia.

She claims she was 'appalled' to find that many of her fellow officers were involved. She was fired by the British company after amassing evidence that UN police were taking part in the trafficking of young women from eastern Europe as sex slaves.

She said: 'When I started collecting evidence from the victims of sex trafficking it was clear that a number of UN officers were involved from several countries, including quite a few from Britain. I was shocked, appalled and disgusted. They were supposed to be over there to help, but they were committing crimes themselves. When I told the supervisors they didn't want to know.'

DynCorp sacked her, claiming she had falsified time sheets, a charge she denies. Last month she filed her case at Southampton employment tribunal alleging wrongful dismissal and sexual discrimination against DynCorp, the British subsidiary of the US company DynCorp Inc.

DynCorp has the contract to provide police officers for the 2,100-member UN international police task force in Bosnia which was created to help restore law and order after the civil war.

Bolkovac has also filed a case against DynCorp under Britain's new Public Interest Disclosure Act designed to protect whistleblowers.

As well as reporting that her fellow officers regularly went to brothels, she also investigated allegations that an American police officer hired by DynCorp had bought a woman for $1,000.

Bolkovac's British lawyers say her evidence will highlight how the underground sex trade in Bosnia is thriving among the 21,000 Nato peacekeepers and thousands of international bureaucrats and aid workers.

Many of the hundreds of women working in Bosnia's sex industry are lured from countries such as Romania and Ukraine with promises of jobs as waitresses but then delivered to brothel owners who confiscate their passports. Bolkovac claims that Dyncorp officers forged documents for trafficked women, aided their illegal transport through border checkpoints into Bosnia and tipped off sex club owners about raids.

In an email to more than 50 people - including Jacques Klein, the UN Secretary-General's special representative in Bosnia - Bolkovac described the plight of trafficked women and noted that UN police, Nato troops and international humanitarian employees were regular customers. It was shortly after this email went out that Bolkovac was reassigned.

Richard Monk, a former senior British policeman who ran the UN police operation in Bosnia until 1999, has sympathy with her plight. He said: 'There were truly dreadful things going on by UN police officers from a number of countries. I found it incredible that I had to set up an internal affairs department to investigate complaints that officers were having sex with minors and prostitutes. The British officers were on the whole extremely good and very professional, setting a great example. But there were policeman from other countries who should not have been in uniform.'

A DynCorp spokeswoman would not comment on the Bolkovac case because it was coming to court later this year. But in a earlier statement the company said: 'The notion that a company such as DynCorp would turn a blind eye to illegal behaviour by our employees is incomprehensible...We encourage our employees to be proactive in reporting inappropriate behaviour and commend those who follow our procedures by reporting it.'

Bolkovac's case is the second against DynCorp alleging misbehaviour in Bosnia. Air mechanic Ben Johnston is suing the company, alleging he was sacked because he had uncovered evidence that Dyncorp employees were involved in 'sexual slavery' and selling arms.

Since 1998 eight Dyncorp employees have been sent back from Bosnia, but none have been prosecuted.